This invention relates generally to meters, and in particular to fluid meters of the oscillating piston or rotary piston type, hereinafter referred to an oscillating piston meters.
The prior art includes a variety of oscillating piston meters. Such meters typically suffer from drawbacks, however, which are either minimized or eliminated in the present invention. One drawback of the prior meters is that the range of flow rates at which acceptable metering accuracy can be maintained is limited by the dynamic instability inherent in the design of such prior art meters. The piston represents an eccentric rotating weight. As the flow rate of the metered fluid increases, the rotational velocity of the piston within the metering chamber increases accordingly. Where the meter comprises a single metering chamber with a single piston therein (such oscillating piston meters hereinafter referred to as single piston meters), the dynamic instability of the meter causes it to vibrate. Such vibration is detrimental to the useful life and accuracy of the meter because it dramatically accelerates wear on the components of the meter, with the concomitant result that the close dimensional tolerances, which appear to be essential to the accuracy of the prior art oscillating piston meters, are not maintained, and internal leakage results. Such vibration is also detrimental to the integrity of the seals which must be maintained at the inflow and outflow connections to the meter. Additional problems resulting from such vibration include energy losses which decrease the hydraulic head of the fluid being metered, and the noise which results from such vibration.
Another disadvantage of the prior art single piston meters is that the acceleration and hence the instantaneous velocity achieved by the piston, upon being subjected to a force generated by the metered fluid, is dependent upon the location of the piston within the metering chamber. This produces a non-linear response for the meter of its output torque to such force, and, hence, output torque and rate of rotation is erratic.
Yet another disadvantage of the prior art single piston meters is that they may have what is known in the art as a dead spot. If the flow of the measured fluid stops, the piston may come to a rest position within the metering chamber such that the sidewall of the piston is against the inflow port to the metering chamber. When the measured fluid attempts to resume flowing, it is difficult, and may in fact be impossible, for the fluid to dislodge the piston from its position against the inflow port.
In some prior art oscillating piston meters attempts have been made to minimize the dynamic instability and other disadvantages associated with single piston meters by including two pistons, substantially 180.degree. out of phase with each other, within the metering chamber. Such meters hereinafter are referred to as dual piston meters. There are several drawbacks associated with the prior art dual piston meters. One disadvantage is that they require complex internal porting between the various components of the meter. Such internal porting increases the number and complexity of the dual piston meter's constituent parts, thereby contributing to sustaining the cost of manufacture and assembly of such meters.
Further, the tortuous path which the metered fluid is required to follow through the prior art dual piston meters decreases their efficiency and also limits the range of flow rates over which acceptable metering accuracy can be maintained. Thus, where the hydraulic head of the metered fluid is low, and the flow rate of the metered fluid is accordingly small, the inefficiency of the prior art dual piston meters may produce inaccurate readings.
Yet another disadvantage of the prior art oscillating piston meters is that they are sized and designed to have a certain maximum flow rate metering capacity. In order to provide oscillating piston meters having a variety of maximum flow rate metering capacities, it is necessary to provide a variety of different size pistons, metering chambers, housings and other constituent parts of the prior art meters. This, in turn, contributes to sustaining the cost of such prior art meters.
A further disadvantage of the prior art oscillating piston meters is that the pistons and other components used therein require several manufacturing steps and also require the maintenance of close dimensional tolerances. (This problem is discussed, in part, in Kullman U.S. Pat. No. 3,587,314, particularly at Column 1, lines 10-28.) The present invention is intended to overcome or minimize these and other disadvantages associated with the prior art oscillating piston meters.